Automatic transmissions have been in use in a variety of vehicle types for some time. They are characterized by a mechanical uncoupling of the gear selection elements and the gear selection in the transmission and enable the driver of the motor vehicle to fulfill his task more easily by either completely automatically selecting and activating a suitable gear or at least checking the driver's gear selection for plausibility and rejecting or modifying it if necessary.
To this end, a decision unit is necessary. The decision unit, with the help of data supplied to it, such as the speed of the vehicle, the input rotational speed of the transmission, motor control data and/or additional data, either selects the gear to be activated or evaluates the gear selection manually entered by the driver and, if and as necessary, modifies it, rejects it or accepts it unchanged as the new gear.
In addition, automatic transmissions are provided with a suitable control device for controlling the automatic transmission, where the control device controls and/or governs the implementation of the actual shifting operation. This device, together with the decision unit, may constitute a single unit. Alternatively, it may take the form of a separate module. Among other things, it may have the ability to exercise a retroactive effect on the decision unit when the target gear cannot be activated.
In applications of this type, the decision unit generally includes a standstill circuit which, when the vehicle is standing still or when its speed falls below a predetermined limit speed, brings about or prepares the activation of a starting gear, which is generally the lowest forward gear. This is advantageous in a normal vehicle operation, however under certain circumstances it may result in an overall undesirable behavior of the automatic transmission.
When a vehicle starts moving under the influence of considerable tractive resistance, for example on a steep slope, under very heavy load, on loose soil or into an extreme headwind in combination with a box body, it happens that after the vehicle has started up and while the automatic transmission is shifting into a higher gear selected by the decision unit or by the driver, the vehicle slows down considerably as soon as the traction force of the drive motor is no longer available to the wheels. The only exceptions to this situation are certain special types of transmissions designed for shifting gears under load.
As a result of the interruption of traction force occasioned by the shifting operation and the concomitant reduction of the vehicle speed, a standstill circuit in the decision unit is activated when, for example, the vehicle speed falls below the predetermined limit speed or is reduced to zero or even to a negative value.
The target gear originally selected by the decision unit, generally the second gear, is then replaced by a gear predetermined by the standstill circuit, most frequently the first gear. This, in turn, leads to an interruption of the original shifting process and an activation of the new target gear or, insofar as the shifting process had already been activated when the standstill circuit was selected, an immediate shift back into the new target gear. Accordingly, as long as the boundary conditions remain unchanged, it will not be possible to shift into the second gear, because the standstill circuit will be reactivated again and again as soon as a shifting operation into a higher gear is initiated or performed.
Various solutions for this problem are conceivable. Each such solution, however, is burdened with specific disadvantages.
Power shift transmissions of the type mentioned above enable the gears to be shifted without interrupting the traction force. As a general rule, however, they are significantly more expensive than other transmissions. Furthermore, they often significantly impose more stringent requirements with regard to the installation space necessary.
A driver in the situation described above might be able, without changes in the transmission or the transmission control, to use the starting gear until a higher vehicle speed is attained, in order to prevent the vehicle from losing speed during the shifting process, to the point of falling below the limit speed and triggering the standstill circuit. The problem with such an action is that, precisely in the case of an automatic transmission, the driver can actually exert, at best, only an indirect influence on the gear selection, for example, by pressing down hard on the gas pedal in order to induce a higher engine rotational speed, thereby delaying the triggering of the shifting process by the decision unit. If the vehicle is starting up on loose ground, such as deep sand or on gravel, snow or ice, in combination with an incline, this can give rise to a significant risk of wheel spin.
This is unacceptable, not only for reasons of safety, but because it causes a sharp increase in the rotational speed of the wheels, while only slightly increasing the vehicle speed. Because most vehicle systems, however, do not utilize the true speed of the vehicle on the ground, but rather calculate the vehicle speed on the basis of a value derived from the rotational speed of the wheels, wheel spin of the driven wheels leads to the triggering of the shifting process, which was, in fact, intended to be delayed by this action until the vehicle achieved a higher speed.
If the vehicle is equipped with a manual gear selection device, the driver can attempt to start in the second gear and thereby avoid the problem described above, at the price of increased wear on the clutch. In any event, though, conventional standstill circuits are made in such a way as to correct such a gear selection by the driver automatically, meaning that this action as well will not be productive
Alternatively, the driver could attempt through the use of shifting elements in the nature of a manually sequentially shiftable shifting device of the automatic transmission or by limiting the shiftable gears by way of a transmission selector lever—to put off shifting gears until the vehicle has achieved a higher speed. This action could be helpful in certain borderline cases, however, such action assumes conscious intervention on the part of the driver and reaches its limits at the latest when the decision unit, on the basis of the data available to it, automatically triggers a shifting process. For this reason, it is not applicable to automatic transmissions which do not comprise elements for the manual selection of a target gear.
In view of that set forth above, it may be stated, as a general rule, that a standstill circuit, though basically expedient in normal vehicle operation, can considerably impede the operation of a vehicle under certain boundary conditions, by preventing, or at least impeding, a shift into a higher gear as the vehicle starts up.
Against the background set forth above, the object of the present invention is to propose a method for controlling a standstill circuit of an automatic transmission, by way of which the disadvantages described above may be avoided. The object is to especially enable a vehicle to start up safely even under the influence of considerable tractive resistance. Furthermore, the object of the invention is to propose a device such that the method can be implemented. Moreover, both the method and the device should be characterized by a solution of the task which is as simple, inexpensive and easy to implement as possible.